For coping with a rapid load change and the like in excess of a power generation ability of a fuel cell in a fuel cell system mounted on a car and the like, there have been suggested various hybrid type fuel cell systems including a fuel cell and a battery as power sources.
In the hybrid type fuel cell systems, an output voltage of the fuel cell and an output voltage of the battery are controlled by a DC/DC converter. As the DC/DC converter which performs such control, there is widely utilized a type of converter which allows a switching element of a power transistor, IGBT, FET or the like to perform a PWM operation, thereby converting the voltage. It has been desirable that the DC/DC converter achieves lower loss, higher efficiency and lower noise with the power saving, miniaturization and performance improvement of an electronic apparatus. In particular, it has been desirable that switching loss and switching surge accompanying the PWM operation are decreased.
One of technologies for decreasing the switching loss and switching surge is a soft switching technology. Here, the soft switching is a switching system for realizing zero voltage switching (ZVS) or zero current switching (ZCS), and the switching loss of a power semiconductor device or stress given thereto is low. On the other hand, a switching system which directly turns on or off a voltage or a current by a switching function of the power semiconductor device is referred to as hard switching. In the following description, a system in which both or one of ZVS and ZCS is realized will be referred to as the soft switching, and another system will be referred to as the hard switching.
The soft switching is realized by a converter (the so-called soft switching converter) in which an auxiliary circuit for decreasing the switching loss is added to a usual step up/down type DC/DC converter including, for example, an inductor, a switching element and a diode (e.g. see Patent Document 1).